This invention relates generally to methods and apparatus for the recovery of oil and gas from underground formations, and more particularly to the recovery of oil from known oil depleted formations and geological formations having properties and characteristics not considered heretofore to be suitable for economic recovery of the oil.
Recovery of oil from underground formations necessarily requires some flow of the oil through the formation to reach a point where it is typically lifted by mechanical means to the earth's surface. It should be readily apparent that the flow rate of the oil, since it is viscous liquid, through the rock formation depends on many factors, which include the hydraulic head of the oil, the thickness and permeability of the formation as well as the density and viscosity of the oil therein.
Many methods have been developed to increase the recovery of oil from underground formations. Typically, these methods have been of the secondary recovery type and utilized only after the collection or production rate of oil falls below an economically acceptable level. Many of the methods have been based on increasing the proximity or exposure of the drainhole path to the petroleum within the oil-bearing formation.
For example, as far back as 1920, Ehrat, in U.S. Pat. No. 1,660,187, described a method of recovering oil and gas from one or more oil-bearing strata in which galleries were excavated underneath the strata and boreholes were drilled upwardly from the galleries into the oil-bearing strata. The galleries were connected to a shaft leading to the earth's surface and all of the boreholes lined with tubes for collecting the oil and gas and for conducting it to the earth's surface by a system of pumps and piping.
This method, however, did not utilize horizontal holes drilled within the oil-bearing strata for the recovery of oil. Horizontal holes are far more effective in the recovery of oil from oil-bearing strata because of the proximity of the drainhole path to the petroleum to be collected.
It is also apparent that a horizontal borehole within an oil-bearing formation is more efficient for collecting oil therefrom than vertical holes or taps, because, in general, more borehole exposure within the oil-bearing formation is possible with the horizontal holes.
Another factor in the economic feasibility in oil recovery methods is the amount and cost of maintaining the collection system established by the method.
For example, maintenance of the Ehrat system is expensive because of the number of tubes, valves and pipes necessary to conduct the oil and gas to the earth's surface. Since the Ehrat galleries are ventilated and personnel must be present to attend the system, additional expense is incurred such as continual safety inspections, mandated by the Bureau of Mines, during the operation of the recovery method.
Ranney recognized the advantages of horizontal drilling for oil within formations, in the June issue of The Petroleum Engineer, in 1939, in an article entitled, "The First Horizontal Oil Well," and in his U.S. Pat. Nos. 2,280,851 issued on Apr. 28, 1942, and 2,365,591 issued on Dec. 19, 1944.
Ranney noted that horizontal wells are far more effective than vertically drilled wells in sands such as the Alberta tar sands. With regard to recovering oil from the Alberta tar sands, Ranney noted that, "something more than the mere drilling of a horizontal well must be resorted to." Ranney thereafter teaches methods for delivering a reagent into oil-bearing formations under conditions to recover the oil disposed about horizontal drill holes.
Not envisioned by Ranney, was the use of horizontal holes in multiple oil-bearing strata to recover or collect oil or the economics of utilizing a plurality of spaced-apart oil-bearing formations in a sealed system to recover oil.
As is apparent in FIG. 2, and within the specification of U.S. Pat. No. 2,365,591, Ranney contemplated a complex valve and piping arrangement for the injection fluids and for the recovery of dissolved oil therefrom. All of this necessitated access to the underground equipment during production and, as hereinabove pointed out, in connection with Ehrat, such an accessible oil recovery system causes a less favorable economic operation of the oil collection system.
Horizontal boreholes were also used in the methods of Turk, described in U.S. Pat. No. 4,160,481 issued on July 10, 1979, and Allen in U.S. Pat. No. 4,257,460 issued on Mar. 21, 1981. Although these methods employed generally horizontal holes drilled into underground oil-bearing formations, they primarily were directed to creating oil-driving forces by the injection of steam and/or inert gas in order to force the oil from the stratum into the horizontal boreholes.
As pointed out in Turk, a number of generally horizontal boreholes may be drilled from a shaft in a radial-like manner into an oil-bearing formation, and these radial horizontal holes may be positioned at a slight upward angle relative to their respective shaft hole in order to accommodate gravity flow of the oil in the formation. The drilling of holes in this manner also facilitated the use of a leaching fluid, which is injected through a portion of each horizontal hole in order to leach the oil therefrom back into the shaft.
It certainly can be appreciated that this leaching operation, whether it be with a solvent, with steam or an inert gas, as described in Allen, requires a significant amount of underground piping and valves, in addition to costly ventilation of the working areas to enable workmen to enter into the shafts and working areas for the purpose of operating the system to recover oil.
Although work has been progressing since 1920 regarding methods of collecting oil and gas from underground formations and particularly from underground formations from which the easily recovered oil has been collected, there has yet to be developed an economical method, or process, for removing oil from known formations having a plurality of spaced-apart oil-bearing strata.
The present invention is directed to a process utilizing horizontal holes drilled in a plurality of spaced-apart subterranean oil-bearing formations.
All of the previous work with the drilling of horizontal holes in underground formations was done at a single level, without any of the researchers realizing the economy in drilling horizontal holes into a plurality of spaced-apart oil-bearing strata.
Further, it has not been recognized that the utilization of a plurality of horizontal holes drilled from a single vertical shaft into a plurality of underground oil-bearing formations can be used as a collection system itself without the need for an extensive and expensive underground piping and valves, which must be maintained and operated by personnel, the latter requiring ventilation throughout the shaft and working areas, which is expensive, adds to the expense, and requires constant safety inspections.
The present invention overcomes all of the problems confronting the prior investigators and provides for a method or process for collecting oil and gas from a plurality of spaced-apart underground oil-bearing formations by the utilization of horizontal holes in a sealed collection system.